1. Field of the Invention
The present invention relates to a cracking-resistant gasket having a function to stably prevent an explosion, a method of molding the gasket, and a cylindrical alkaline-manganese dioxide cell incorporating the gasket to provide an excellent leakage-proof structure.
2. Description of Related Art
If a cylindrical alkaline-manganese dioxide cell is overcharged or over-discharge due to an incorrect use, gases are generated furiously inside the container or can of the cell to elevate the internal pressure, possibly resulting in a leakage of the electrolyte, etc. and in an explosion in some cases. To avoid this, the cylindrical alkaline-manganese dioxide cell is provided with an explosion-proof mechanism to quickly purge gases having been generated inside the cell and elevated the internal pressure.
Generally, the above-mentioned explosion-proof mechanism adopted in the conventional alkaline-manganese dioxide cells is a thinned portion of a gasket sealing the opening of the cell can and which acts as an explosion-proof safety valve. More specifically, a stress caused by an internal pressure elevated by gases generated inside the cylindrical alkaline-manganese dioxide cell is concentrated to the gasket whose thinnest portion will be torn. The gases generated inside the cell are purged to outside through an opening thus formed in the gasket, thereby preventing the cell from exploding due to an elevated internal pressure inside the cell.
There have so far been proposed various types of conventional gaskets formed in various shapes, by molding, from a synthetic rein such as Nylon, polypropylene, etc. or a rubber as shown in FIGS. 1 to 4.
FIG. 1 illustrates a first example of conventional gasket in the form of an axial sectional view. The gasket is generally indicated with a reference numeral 50. It consists of a thick-walled cylindrical portion 51 formed at the center thereof and having a convex axial-section (will be referred to as xe2x80x9chob portionxe2x80x9d hereinunder), a disc-like portion 52 formed along the outer circumference of the hob portion 51 and concentrically with he hob portion 51 (will be referred to as xe2x80x9cdiaphragm portionxe2x80x9d hereinunder), thin portions 53 formed between the hob and diaphragm portions 51 and 52 and in a position nearer to the upper side of the diaphragm portion 52, and a peripheral portion 54 formed contiguously along the outer circumference of the diaphragm portion 52 to have the axial-sectional shape of a curve. The hob portion 51 is a cylinder having a hob hole 55 open at the top thereof as indicated with a reference numeral 55a and at the bottom thereof as indicated with a reference numeral 55b and whose inside diameter is gradually increased downward so that the lower opening 55b of the hob hole 55 is larger than the upper opening 55a. The thin portions 53 are two arcs each formed concentrically with the hob portion 51 as shown in FIG. 5.
FIG. 2 shows a second example of conventional gasket in the form of an axial sectional view. This gasket is generally indicated with a reference numeral 60. It consists of a hob portion 61, diaphragm portion 62, thin portions 63 and a peripheral portion 64 as in the gasket 50 having been described above. In this example, the hob portion 61 is a cylinder having a hob hole 65 whose lower portion is larger in diameter than the upper portion. That is to say, the wall of the hob hole 65 is stepped at the boundary between the upper and lower portions. As shown in FIG. 5, the thin portions 63 are two arcs concentric with the hob portion 61 similarly to the thin portions 53 of the gasket 50.
FIG. 3 is an axial sectional view of a third example of conventional gasket. The gasket is generally indicated with a reference numeral 70. The gasket 70 consists of a hob portion 71 formed to have a convex section, a diaphragm portion 72 formed concentrically with the hob portion 71 and obliquely towards the upper end portion of the hob portion 71, thin portion 73 formed between the hob portion 71 and diaphragm portion 72 in a position nearer to the upper side of the diaphragm portion 72, and a peripheral portion 74 formed contiguously along the outer circumference of the diaphragm portion 72 to have the axial-sectional shape of a curve. The hob portion 71 is formed longer than those 51 and 61 of the gaskets 50 and 60, respectively, to project downward from the bottom surface of the diaphragm portion 72. The hob portion 71 is a cylinder having a straight bob bole 75 having a same diameter everywhere through it. The thin portion 73 takes the form of a circle formed concentrically with the hob portion 71 as shown in FIG. 6.
FIG. 4 shows a fourth example of conventional gasket in the form of an axial sectional view. The gasket is generally indicated with a reference numeral 80. The gasket 80 includes a hob portion 81, a diaphragm portion 82 formed concentrically with the bob portion 81, a thin portion 83 formed in a portion of the diaphragm portion 82, and a peripheral portion 84 formed contiguously along the outer circumference of the diaphragm portion 82. The bob portion 81 consists of upper and lower halves projecting upward and downward, respectively, to a generally same extent from the diaphragm portion 82a. The bob portion 81 is a cylinder having formed therein a straight bob hole 85 open at the top thereof as indicated with a reference numeral 85a and at the bottom thereof as indicated with a reference numeral 85b and having a same diameter everywhere through it. The thin portion 83 is formed to extend in a round hole in a portion of the diaphragm portion 82 as shown in FIG. 7.
The aforementioned gaskets 50, 60, 70 and 80 (will be generically called xe2x80x9cgasket 50xe2x80x9d hereinunder) are formed by an injection molding in which a molten resin is injected from an injection gate into a space defined between male and female molds. FIG. 8 is an axial sectional view of a first example of mold assembly for molding the conventional gaskets. The mold assembly is generally indicated with a reference numeral 90. Referring to FIG. 8, how to mold the gasket using the mold assembly 90 will be described herebelow. The mold assembly 90 is a combination of a male mold 90a and a female mold 90b. When set in place, the male and female molds 90a and 90b define together a cylindrical space 91 for the hob portion, a flat space 92 for the diaphragm portion, a curved space 93 for the peripheral portion, and a narrower space 94 contiguous to the inner end of the flat space 92 to form the thin portion 94. The female mold 90b has a pin point gate 96 formed therein in the proximity of the base of a core pin 95. The pin point gate 96 is open at the bottom of the cylindrical space 91. The gasket 50 is molded by injecting a molten resin into the mold assembly 90.
FIG. 9 shows, in the form of an axial sectional view, a second example of mold assembly used for molding the conventional gaskets. The mold assembly is generally indicated with a reference numeral 100. The mold assembly 100 is a combination of a male mold 100a and a female mold 100b. Different from the mold assembly 90 shown in FIG. 8, the mold assembly 100 is of a gate type for molding the gasket 50. As in the gate 90, the gate 100 has a cylindrical space 101, a flat space 102, a curved space 103, and a narrower space 104. By injecting a molten resin into the mold assembly 100 from a side gate 105 open at the end of the curved space 103, the gasket 50 is molded.
The gate 50 can be formed using any one of various mold assemblies having so far been proposed and which are provided with a film gate or disc gate formed to extend from the bottom of the cylindrical space for the hob portion, a single- or multi-point pin gate formed to extend from the flat space for the diaphragm portion, and a submarine gate formed to extend from the curved space for the peripheral portion.
For use in the cylindrical alkaline-manganese dioxide cells, there have been proposed the gaskets molded to have various shapes as mentioned above, and also various mold assemblies destined for such gasket shapes and various gates formed in the mold assemblies have been proposed.
However, if a mold design and gate formed in the mold are not suitable for a gasket to be produced by an injection molding, the molten resin will not flow in a desired direction and air in the mold assembly will not sufficiently be purged. In this case, the molten resin will not evenly be filled into the mold assembly and the gasket will be molded from the insufficient amount of the molten resin thus filled. In the resulting gasket, void, sink or weld mark will occur at the hob portion to weaken the gasket, the thin portion will incur a weld mark, and thus the gasket will have no uniform distribution of strength.
In a gasket thus molded with such defects, a shock given when a current collector is inserted into the hob portion, a stress developed in the hob portion after the current collector has been inserted, a long-term storage at room temperature or storage at undesirable temperatures, will cause the hob portion to easily crack. If such a crack takes place at the hob portion, the sealing between the hob portion and current collector will be broken and thus an electrolyte in the cell will move between the hob portion and current collector until it leaks from the cell. Also, such a cracking of the hob portion will not lead to a uniform, stable tearing of the thin portion which acts as an explosion safety valve.
In the cylindrical alkaline-manganese dioxide cells, the cracking of the gasket hob portion and nonuniform tearing of the thin portion is a great cause of the electrolyte leakage from the cell.
Further, it has been a great technical difficulty to mold a gasket incurring no void, sink or weld mark at the hob portion, no weld mark at the thin portion and no nonuniform distribution of strength and which can thus keep the cell against leakage of electrolyte.
Accordingly, the present invention has an object to overcome the above-mentioned drawbacks of the prior art by providing a gasket excellent in cracking resistance and tearable uniformly and stably to serve as an explosion safety valve, a method of molding the gasket, and a cylindrical alkaline-manganese dioxide cell using the gasket and in which electrolyte leakage can effectively be prevented
According to the present invention, there is provided a gasket including a cylindrical portion whose lower portion is gradually increased in thickness towards the lower end thereof, having formed therein a hole open at the top and bottom thereof, the lower opening being larger than the upper one, and in which a current collector is to be inserted; a disc-shaped portion formed contiguously along the outer circumference of and concentrically with the cylindrical portion; a thin portion formed between the cylindrical and disc-like portions, concentrically with the cylindrical portion and in a position nearer to the lower side of the disc-shaped portion; and a peripheral portion formed contiguously along the outer circumference of the disc-shaped portion to have the axial-sectional shape of a curve; the cylindrical portion being formed to project at the lower end thereof downward from the bottom of the disc-shaped portion; the boundary between the cylindrical and thin portions being chamfered or curved; and the boundary between the disc-shaped and thin portions being formed like a generally vertical edge.
According to the present invention, there is provided a method of molding a gasket using a mold assembly consisting of a male mold and female mold which will define together, when being assembled to each other, spaces into which a resin is filled to mold the gasket, the spaces including a cylindrical space; a flat space positioned along the outer circumference of the cylindrical space; a curved space contiguous to the outer circumference of the flat space; and a narrow space resulted from a projection formed on the male mold between the cylindrical and disc-shaped spaces, the projection being chamfered or curved at a side thereof facing the cylindrical space and directed downward in the direction of the female mold; the male mold having formed therein an injection gate open at the top of the cylindrical space; and at least any one of an ejector sleeve formed on the female mold at the bottom of the cylindrical space and an ejector pin formed in the curved space, being provided for the resin to be filled uniformly into the mold assembly.
According to the present invention, there is provided a cylindrical alkaline-manganese dioxide cell including a can open at one end thereof, the can opening being sealed with a gasket including a cylindrical portion whose lower portion is gradually increased in thickness towards the lower end thereof, having formed therein a hole open at the top and bottom thereof, the lower opening being larger than the upper one, and in which a current collector is to be inserted; a disc-shaped portion formed contiguously along the outer circumference of and concentrically with the cylindrical portion; a thin portion formed between the cylindrical and disc-like portions, concentrically with the cylindrical portion and in a position nearer to the lower side of the disc-shaped portion; and a peripheral portion formed contiguously along the outer circumference of the disc-shaped portion to have the axial-sectional shape of a curve; the cylindrical portion being formed to project at the lower end thereof downward from the bottom of the disc-shaped portion; the boundary between the cylindrical and thin portions being chamfered or curved; and the boundary between the disc-shaped and thin portions being formed like a generally vertical edge.